1. Field of the Invention
This invention generally relates to a low-power-consumption source driver, and more particularly to a low-power-consumption source driver for a liquid crystal display (LCD).
2. Description of Related Art
FIG. 1 is a structure diagram of a LCD. Referring to FIG. 1, the LCD uses the thin film transistor 100 as a switch. When the gate driver 104 outputs signals to turn on the thin film transistor 100, the source driver 102 will output image data to the liquid crystal, and the liquid crystal change its status according to image data.
FIG. 2 is a block diagram of the source driver of FIG. 1. Referring to FIG. 2, the source driver 102 of the LCD comprises a shift register 200, a latch 202, a level shifter 204, a digital-to-analog converter (DAC) 206 and an output buffer 208. The shift register 200 sequentially writes digital image data into the latch 202. When image data stored in the latch 202 are enough to display a horizontal line, the latch 202 will output image data to the level shifter 204. The level shifter 204 changes the voltage level of digital image data and then outputs image data to the DAC 206. The DAC 206 receives digital image data and then outputs analog image data to the output buffer 208. Finally, the output buffer 208 writes image data to the liquid crystal. The output buffer 208 is constructed by a unit-gain and negative-feedback operational amplifier.
To prevent the liquid crystal from the ion effect, the polarity of the voltage signals applied on the liquid crystal has to be changed continuously. Hence, a portion of the driver circuit, such as the DAC and the output buffer, are classified into the positive and negative types, for example, the positive analog circuit 306 and the negative analog circuit 308 of FIG. 3. Traditionally, the driving voltages for the positive and negative driver circuits are the same. However, the range of the driving voltages should be different for the positive and negative driver circuits.
To make sure that the circuits with different polarities can operate properly, traditionally the range of operational voltage is twice larger than that of the driver circuit with the single polarity. The traditional method has the following drawbacks:
1. The First and second level shifters 302 and 304 will raise the voltage of the input signal to the same voltage. When the voltage level of the input signal is changed, it will increase the power consumption (P=f*C*V2). For example, if the voltage is increased by twice, the power consumption will be increased by four times.
2. When the DACs with the different polarities use the same operational voltage, it also increases the dynamic power consumption under the consideration of the parasitic capacitors Cgs and Cgd.
3. When the output buffers with different polarities use the same operational voltage, it also increases the static power consumption (P=I*V). For example, if the voltage is increased by twice, the static power consumption will be increased by twice.
To reduce the power consumption, the present invention discloses a low-power-consumption source driver to reduce the amplitude of the operational voltage of the level shifters and the analog circuits with different polarities. Hence, the present invention can reduce the power consumption and thus further reduce the cost of the circuit.